1. Field of the Present Description
The present invention relates generally to the field of flight control systems for aircraft and relates particularly to a system for automatic flight and recovery in microbursts.
2. Description of Related Art
An unmanned aerial vehicle (UAV) is defined as a powered, aerial vehicle that has does not carry a human operator, flies autonomously or is piloted remotely, and is expendable or recoverable. When control is exercised by a remote pilot, control may be continuous or episodic. However, autonomous vehicles may follow preprogrammed courses and may or may not have the capacity for rerouting or re-targeting.
UAVs are typically designed to be reusable, though UAVs have shorter expected life spans than manned aircraft due to higher attrition in operations, especially in combat. UAVs will survive for a relatively small number of sorties until failures, accidents, or hostile action destroys them. The loss rate for aircraft and UAVs is an important concept that influences the cost-effectiveness of UAVs and manned vehicles.
A principal reason for using UAVs is not only to reduce the risk to humans in combat or other hazardous missions, but also to perform missions in a more efficient and less costly fashion than has historically been the case with manned vehicles. Another related reason is that freeing machines from the limitations imposed by humans would allow an increase in their performance. From the beginning, the hope has been that unmanned air vehicles would be less expensive to develop and manufacture than that of manned aircraft, and that UAVs will reduce the demand for the supporting facilities and manpower that modern manned aircraft require.
As a result of technological advances in flight control, data and signal processing, off-board sensors, communications links, and integrated avionics, UAVs are now a serious option. To allow for autonomous operation, many types of systems are being developed for UAVs, including, for example, systems for friend-or-foe recognition or traffic avoidance. Flight control systems may be configured for controlling UAVs in particular types of flights, such as search-and-rescue operations, or for flight during inclement weather conditions.
In manned aircraft, a pilot is in control of the aircraft and will take corrective action when weather conditions deteriorate and cause a safety concern. For example, a pilot may see a thunderstorm ahead of the aircraft and steer the aircraft around the storm to avoid the undesirable effects of low-level windshear in the form of microbursts. However, a UAV operating autonomously will not know the storm is approaching and may fly directly into the storm.
The effects of microbursts on aircraft have been a subject of intensive investigation for the past several decades. Microbursts are characterized by a downward gust of wind that produces sudden changes in wind speed or its direction when it interacts with the ground. Microbursts are considered a serious hazard to flight safety, producing large adverse aerodynamic effect on aircraft performance, changing the aircraft flight path without warning, and impairing and thrust, momentum and lift of an aircraft. Since many aircraft accidents have been attributed to the effect of low-level windshear, design of control systems for aircraft encountering microburst has become an important topic in flight control.
The adverse aerodynamic effects on the aircraft due to microbursts result from a sudden and substantial change in the vertical and horizontal wind speed. Microburst models have been developed using in-flight data, and most of these models employ the Dryden representation of wind gusts. These models are combined with the aircraft parameter identification method to develop the necessary control laws. One of main difficulties in designing the control laws has been the uncertainty in modeling of aerodynamic coefficients for microburst conditions and the ability to account for nonlinear effects.
When a UAV flies toward a microburst range, the controlling ground station may not detect this possible hazard. At the same time, the UAV may lose the communications link with the ground station and may not be able to receive other satellite or other wireless related signals, such as Global Positioning System (GPS) signals. How the UAV reacts to the conditions within the microburst will determine whether the UAV can survive under microburst attack, and it has been shown that standard flight control laws may not successfully direct the UAV to escape the microburst.
The problem of controlling the flight and recovery of a UAV under microburst conditions on the flight path is shown in FIG. 1. As shown in the figure, UAV 11 is programmed to fly along path 13. A microburst area 15 in storm 17 produces a central downburst 19, which is directed outward into lateral outbursts 21, 23 when downburst 19 contacts the ground below storm 17. As UAV 11 passes through outburst 21, it may have extra lift generated by the upward wind coming from the ground or by a head wind, and this condition will be referred to as Condition 1. UAV 11 in this condition is “ballooning,” and the extra lift may cause the flight control system to reduce the flight control inputs due to this phenomenon. UAV 11 will then lose large amounts of lift when the direction and speed of winds change as UAV 11 flies into Condition 2, which is the central downburst 19.
When UAV 11 is in Condition 2, the downward rushing wind (which may have heavy rain entrained within) directly impacts the aircraft on the body, wings, and tail. UAV 11 will lose some lift due to this effect if the flight control system does not make any correction. From Condition 1 to Condition 2, the lift of UAV 11 is slightly increased in Condition 1 and suddenly decreases a large amount in Condition 2. This unexpected change will not be recognized by the system immediately, and this is one of the reasons that aircraft have greatly diminished performance during this transition.
As UAV 11 enters outburst 23, which forms Condition 3, the lift of UAV 11 is still decreasing. A tail wind causes the wings to lose lift, and the downward wind pushes UAV 11 toward the ground. In a helicopter or other rotary-wing aircraft, this kind of motion may impair the main rotor and flight control. If the control system does not make any correction in Condition 2, at Condition 3 it may be too late for the UAV 11 to recover from the microburst effect. Therefore, design of a robust control law to offset the effects of all three conditions may allow UAV 11 to survive a microburst event.
Typically, the tools used to provide the necessary information for aircraft in windshear prediction are a weather radar and an electronic flight instrument system (EFIS). The weather radar, air data computer (ADC), or other sensor works on an echo principle for weather detection and ground mapping. The radar/ADC sends out short bursts of electromagnetic energy that travel through space as a radio wave. When the traveling wave of energy strikes a target, some of the energy reflects back to the radar receiver, and electronic circuits measure the elapsed time between the transmission and the reception of the echo to determine the distance to the target (range). The EFIS system works with the flight management system to provide the necessary information for the flight control. This information may include wind speed and direction, altitude, pitch, pitch rate, angle of attack, or other parameters. Therefore, combination of the weather radar/ADC and EFIS system will provide the information needed by the aircraft.